Today's computer networking environments, such as the Internet, offer mechanisms for delivering documents between heterogeneous computer systems. One such network, the World Wide WEB network, which comprises a subset of Internet sites, supports a standard protocol for requesting documents known as WEB pages and for receiving them. This protocol is known as the Hypertext Transfer Protocol, or "HTTP." HTTP defines a high-level message passing protocol for sending and receiving packets of information between diverse applications. Details of HTTP can be found in various documents including T. Berners-Lee et al., Hypertext Transfer Protocol--HTTP 1.0, Request for Comments (RFC) 1945, MIT/LCS, May, 1996, which is incorporated herein by reference. Each HTTP message follows a specific layout, which includes amongst other information a header, which contains information specific to the request or response. Further, each HTTP request message contains a universal resource identifier (a "URI"), which specifies to which network resource the request is to be applied. A URI is either a Uniform Resource Locator ("URL") or Uniform Resource Name ("URN"), or any other formatted string that identifies a network resource. The URI contained in a request message, in effect, identifies the destination machine for a message. URLs, as an example of URIs, are discussed in detail in T. Berners-Lee, et al., Uniform Resource Locators (URL), RFC 1738, CERN, Xerox PARC, Univ. of Minn., December, 1994, which is incorporated herein by reference.
FIG. 1 illustrates how a browser application enables users to navigate among nodes on the WEB network by requesting and receiving WEB pages. For the purposes of this application, a WEB page is any type of document that abides by the HTML format. That is, the document includes an "&lt;HTML&gt;" statement. Thus, a WEB page is also be referred to as an HTML document. The HTML format is a document mark-up language, defined by the Hypertext Markup Language ("HTML") specification. HTML defines tags for specifying how to interpret the text and images stored in an HTML document. For example, there are HTML tags for defining paragraph formats and for emboldening and underlining text. In addition, the HTML format defines tags for adding images to documents and for formatting and aligning text with respect to images. HTML tags appear between angle brackets, for example, &lt;HTML&gt;. Further details of HTML are discussed in T. Berners-Lee and D. Connolly, Hypertext Markup Language-2.0, RFC 1866, MIT/W3C, November, 1995, which is incorporated herein by reference.
In FIG. 1, a WEB browser application 101 is shown executing on a client machine 102, which communicates with a server machine 103 by sending and receiving HTTP packets (messages). Web browser 101 "navigates" to new locations on the network to browse (display) what is available at these locations. In particular, when WEB browser 101 "navigates" to a new location, it requests a new document from the new location (e.g., server machine 103) by sending an HTTP-request message 104 using any well-known underlying communications wire protocol. HTTP-request message 104 follows the specific layout discussed above, which includes a header 105 and a URI field 106, which specifies the network location to which to apply the request. When the server machine specified by URI 106 (e.g., server machine 103) receives the HTTP-request message, it decomposes the message packet and constructs a return message packet to the source location that originated the message (e.g., client machine 102) in the form of an HTTP-response message 107. In addition to the standard features of an HTTP message, such as the header 108, the HTTP-response message 107 contains the requested HTML document 109. When the HTTP-response message 107 reaches the client machine 102, the WEB browser application 101 extracts the HTML document 109 from the message, and parses and interprets (executes) the HTML code in the document in order to display the document on a display screen of the client machine 102 as specified by the HTML tags.
To provide additional security in a global network environment, some WEB browser applications incorporate the use of an intermediary machine between the client machine and other machines on the WEB. For example, several client machines may be networked internally to a proxy server machine, which acts as a "firewall" between the client machines and server machines on an external network. FIG. 2 illustrates the use of a proxy server machine to send and receive HTTP documents. The WEB browser application 201, which is shown executing on a client machine 202, sends an HTTP-request message 205 to a server machine 203. The server machine (e.g., server machine 203) is specified by the URI field 206 contained in the HTTP-request message 205. However, in contrast to FIG. 1, the HTTP-request message 205 is sent first to a proxy server machine 204, which then forwards the HTTP-request message 205 as message 207 to the server machine 203. Because the proxy server machine 204 contains a separate external network connection to server machine 203, the client machine 202 is protected from ill-behaved applications, which may be executing on or accessible via server machine 203. For example, the proxy server machine may be specially programmed to detect viruses that may be sent in an HTTP-response message. When the HTTP-request message 207 is received, server machine 203 sends an HTTP-response message 208, which contains the requested HTML document 209, to the originator of the message. This time, the originator of the message is the proxy server 204. Thus, the HTTP-response message 208 is first sent to the proxy server machine 204 before the requested document is delivered to client machine 202. The proxy server machine 204, by having previously established a specific network connection for the original HTTP-request message 205 received from the client machine 202, knows that the received HTTP-response message 208 corresponds to the HTTP-request message 205 and thus forwards the HTTP-response message 208 as HTTP message 210 to the client machine 202. The WEB browser application 201 is then responsible for decomposing the HTTP-response message 210 to extract the HTML document and for executing the HTML code in the received document to properly display the document on client machine 202.
In network environments such as those shown in FIGS. 1 and 2, WEB browser applications, such as WEB browser 101, have attempted to make the navigational process easier for users by adding various "links" to places of interest. These links can be used to navigate to a pre-specified location by simple maneuvers, such as clicking on a graphical button on a screen. A link in this context refers to a URI, which is used by the browser application to locate another document. For example, a WEB browser may provide graphical buttons, which are placed below the standard menus in toolbars. These buttons provide links that enable a user to easily navigate to specific locations defined by the developer of the WEB browser. In order to ensure that such links are present in every WEB page displayed, the browser application must include special code to display these graphical button user interface components. Thus, when a user invokes a different WEB browser application, the user may not have access to links to which the user has become accustomed.